Method and apparatus for starting an induction motor

ABSTRACT

A starting circuit for an induction motor is provided whereby a stationary rotor flux is established then a stored charge supplied to a motor winding in order to provide a starting torque. Before applying a mains power supply to the motor after the starting torque has been applied, a back EMF is sensed, directly or indirectly, to provide an indication that the rotor is turning before the mains supply is connected to the motor. This provides a reliable and simple method for ensuring that a mains supply may be safely connected to the motor.

FIELD OF THE INVENTION

This invention relates to a starting circuit for an electric inductionmotor, and in particular relates to a circuit and method for ensuringthat the motor is turning properly before a full mains supply is appliedto the windings of the motor.

BACKGROUND

It is well known that starting induction motors can cause a number ofproblems. One of the most significant is the large amount of inrushpower that can be required to start the motor. During start up phase,induction motor will draw currents, which exceed the usual currentsupplied to the motor when it is running at full load. The very highcurrent demand can have a detrimental effect upon the local electricitysupply, for example by causing a “brown out” due to the reduction in thesupply voltage.

These problems associated with starting induction motors are set forthin published international application no. WO97/30509 (McDonald),together with a circuit and method for effectively overcoming theproblem of current surges associated with starting induction motors.

The solution proposed by McDonald is to provide a stag circuit for amulti-phase motor whereby a unidirectional current derived from themains supply is supplied between a first combination of the windingterminals of the motor. This establishes a stationary rotor flux in therotor. At the same time as the stationary rotor flux is beingestablished, a capacitor is charged from the mains supply. When thestationary rotor flux has been established, and when the capacitor hasboth charged, the supply of unidirectional current to the firstcombination of terminals is terminated and the charge on the capacitoris applied to a second combination of terminals. The second combinationof terminals is selected to generate a stator flux at an angle between 0and 180 degrees to the stationary rotor flux. Therefore, when the chargeon the capacitor is applied to the second combination of terminals, therotor will attempt to move into the desired position governed by theorientation of the fields. A very high voltage can be built up upon thecapacitor, and therefore a correspondingly high current may be providedin the winding between the second combination of terminals, so a verysignificant and substantial starting torque can been applied to themotor to initiate rotation. Once rotation has been initiated in thisfashion, then the mains supply can be connected to the terminals of themotor in the known way to continue the rotation of the motor.

Therefore, the starting circuit described in McDonald has thesignificant advantage of being capable of being arranged to provide avery high starting torque without drawing a correspondingly high surgecurrent from the mains supply during start up.

A significant disadvantage with the circuit described in McDonald isthat there is no easily measured yet accurate and reliable way ofdetermining whether or not the machine has actually started after thecharge from the capacitor has been applied to the second combination ofterminals. Clearly, if the motor has not started, ie. if the rotor hasnot started rotating, then applying the fill mains supply to the motorcan result in destruction of the motor. Such situations may often occur,for example when the rotor becomes locked or something in the circuitryis faulty so that the rotor does not start turning when the charge onthe capacitor is applied to the second combination of terminals. Thecondition is worse with a single phase supply as there is then nonatural rotation of the supply voltage.

In McDonald, the initiation of the connection of the mains supply to themotor terminals occurs when any of the following three events happens:

-   1. If the current delivered by the capacitor to the second    combination of terminals falls to zero before a predetermined time    period elapses, or-   2. If the predetermined time period elapses before the current    delivered by the capacitor falls to zero, or-   3. If the rate of change of current from the capacitor goes to zero    before the predetermined time period elapses.

Therefore, the strategy is not dependent upon any indication as towhether the rotor is turning, so there is no indication as to when it issafe to connect the supply to the motor.

OBJECT OF INVENTION

Accordingly, it is an object of the present invention to provide astarting circuit, or a method of starting an induction motor, which willat least go some way toward overcoming the foregoing disadvantages, orto at least provide the public with a useful choice.

SUMMARY OF THE INVENTION

In one aspect the invention may broadly be said to consist in a startingcircuit for an electric induction motor having a plurality of phasewindings to be energised from an electricity supply, the phase windingshaving at least three terminals for connection to the supply, thecircuit comprising

a first switching means arranged and controllable to conduct auni-directional current derived from the supply between a firstcombination of the terminals to establish a stationary rotor flux in therotor of the motor,

a second switching means arranged and controllable to supply a startingcurrent between a second combination of the terminals selected togenerate a stator flux at an angle to the stationary rotor flux,

switch control means to control the first switch means to establish thestationary rotor flux and to control the second switching means toinitiate supply of the starting current to provide a starting torque forthe motor, and

back EMF sensing means to sense a back EMF generated by rotation of therotor to sense whether the rotor has turned sufficiently to be in acondition to connect the motor to the supply.

In a further aspect the invention may broadly be said to consist in amethod of starting an electric induction motor having a plurality ofphase windings, the method comprising the steps of

delivering a uni-directional current to the motor to establish astationary rotor flux in the rotor of the motor,

delivering a starting current to the motor to produce a stator flux atan angle to the stationary rotor flux to produce a motor startingtorque,

detecting the back EMF produced by rotation of the rotor in response tothe staring torque, and

determining whether one or more characteristics of the detected back EMFindicates that the motor is in a condition for connection to anoperating supply.

In a further aspect the invention may broadly be said to consist in astaring circuit for an electric asynchronous motor, the circuitincluding a detection means to detect a voltage or current generated byrotation of the rotor during starting for the purpose of ascertainingwhether the motor is in a condition to complete starting the motor.

In a further aspect the invention may broadly be said to consist in astarting circuit for an electric asynchronous motor, the circuitincluding a measuring means to measure a time period from the initiationof rotation for the purpose of ascertaining whether the motor is in acondition to complete starting of the motor.

Preferably the detection means includes measuring means to provide anindication of the magnitude of the detected voltage or current.

Preferably measuring the time period means measuring the time from themoment of initiation of rotation of the rotor to the moment of detectingthe first zero crossing of motor back EMF in a selected stator windingof the motor.

Preferably the circuit includes comparing means to compare the magnitudeof the detected/measured voltage, current or time period with apredetermined voltage/current or time period, the predeterminedvoltage/current or time period being indicative of the motor being inthe condition to complete starting.

Preferably the predetermined voltage, current or time period isindicative of rotational speed of the motor.

Preferably the circuit includes means to complete starting of the motorif the magnitude of the detected voltage or current equals or exceeds apredetermined voltage or current, or the measured time period is lessthan the predetermined time period.

Preferably the detected voltage comprises the back EMF generated in astator winding by rotation of the rotor.

Preferably the motor is an induction motor having a plurality ofwindings to be energised from an electricity supply.

Preferably the circuit includes switching means arranged andcontrollable to supply a current to a first winding to generate astationary rotor flux.

Preferably the switching means is also arranged and controllable tosupply a starting current selected to generate a stator flux, which isselected to ordinarily cause rotation of the rotor.

Preferably the stator flux is generated at 60 or 120 degrees to thestationary rotor flux.

Preferably the electricity supply is connected to the motor if the backEMF equals or exceeds the predetermined value or the measured timeperiod is below the predetermined value.

In a further aspect the invention may broadly be said to consist in amethod of starting an electric asynchronous motor, the method includingdetecting a voltage or current generated by rotation of the rotor ormeasuring a time period from the initiation of rotation of the rotorduring starting for the purpose of ascertaining whether the motor is ina condition to complete starting the motor.

Preferably the step or detection includes providing an indication of themagnitude of the detected voltage, current or the time period.

Preferably the method includes the step of comparing the magnitude ofthe detected voltage or current with a predetermined voltage or current,the predetermined voltage or current being indicative of the motor beingin the condition to complete starting.

Preferably the method includes the step of comparing the magnitude ofthe mere time with a predetermined time period, the predetermined timeperiod being indicative of the motor being in the condition to completestarting.

Preferably the method includes the step of completing starting of themotor if the magnitude of the detected voltage or current equals orexceeds the predetermined value or if the measured time period is lessthan the predetermined value.

Preferably the method includes supplying a starting current selected togenerate a stator flux which is selected to ordinarily cause rotation ofthe rotor.

Preferably the method includes the step of connecting the electricitysupply to the motor if either the back EMF (or the capacitor voltage)equals or exceeds the predetermined value or the measured time period isless than the predetermined value.

Preferably the method includes the step of discontinuing staring of themotor if the back EMF or capacitor voltage or time period measurementsindicate that the motor is not in a condition for starting.

Preferably the method includes the step of reinitiating the startingsequence if the back EMF or time period measurement indicates that themotor is not in a condition for starting.

In a further aspect the invention may broadly be said to consist in astarting circuit for an electric asynchronous motor, the circuitincluding a measuring means to measure a time period from the initiationof rotation for the purpose of ascertaining whether the motor is in acondition to complete starting of the motor.

In a further aspect the invention may broadly be said to consist in astarting circuit for an electric asynchronous motor having a pluralityof windings to be energised from an electricity supply, the circuitincluding;

-   -   a switching means arranged and controllable to establish a        stationary rotor flux in the rotor of the motor, and capacitor        storage means to supply a starting current selected to generate        a stator flux at an angle to the stationary rotor flux which is        selected to ordinarily cause rotation of the rotor, and    -   means to measure the back EMF or the capacitor voltage or the        time required by the back EMF to reach a predefined value caused        by the rotation of rotor and compare the back EMF or capacitor        voltage or time with a predetermined value, and    -   if the back EMF or the capacitor voltage reaches or exceeds the        predetermined value or the measured time is below the        predetermined value, initiate connection of the mains supply to        the motor.

In a further aspect the invention may broadly be said to consist in amethod of starting an electric induction motor having a plurality ofphase windings, the method comprising the steps of;

-   -   delivering a controlled unidirectional current to the motor to        establish a stationary flux in the motor,    -   delivering a starting current from a capacitor to the motor to        produce a stator flux, which will ordinarily turn the rotor,    -   measuring the back EMF or the capacitor voltage or the time        required to reach a predetermined value of back EMF in a        selected phase winding in order to determine whether the motor        is in a condition for application of mains supply, and    -   if the back EMF or capacitor voltage or time measurement        indicates that the motor is in a condition for application of        mains supply, initiating connection of mains supply to the        motor.

Preferably the method includes the step of discontinuing starting of themotor if the back EMF or capacitor voltage or time measurement indicatesthat the motor is not in a condition for starting.

Preferably the method includes the optional step of reinitiating thestarting sequence if the back EMF measurement indicates that the motoris not in a condition for starting.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

The invention consists of the forgoing and also envisages constructionsof which the following gives examples.

One preferred form of the present invention will now be described withreference to the accompanying drawings in which;

DRAWING DESCRIPTION

FIG. 1 is a simplified circuit diagram of a staring circuit connected toa three phase motor for operation from a single phase AC supply.

FIG. 2 is an illustrative graph of current through the second switch,voltage across the capacitor C1 and back EMF across the winding 6-10, ofthe circuit of FIG. 1 plotted against time.

FIG. 3 is an illustrative graph of voltage across the winding 6-10 ofthe circuit of FIG. 1 plotted against time.

FIG. 4 is part of a simplified logic diagram of switching controlaccording to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

A simplified starting circuit shown connected to a three phase electricinduction motor for operation from a single phase supply is shown inFIG. 1, in essence being a simplified diagram of the circuit describedand illustrated in more detail in WO97/30509 (McDonald) which isincorporated herein by reference. A simplified diagram has been used tosimplify the description of operation of the circuit, which may beunderstood in greater detail with reference to McDonald. However, forthe purposes of providing a basic illustration of operation of thestarting circuit, a description is briefly provided below.

Turning to FIG. 1, a unidirectional ie. a DC supply 2 (which ispreferably derived from mains supply 4) can be connected across a firstcombination of terminals 6 and 8 via a switch S1 (which may comprise atransistor for example). Closing switch S1 provides a unidirectionalcurrent to the winding between terminals 6 and 8 and generates astationary flux in the rotor of the motor.

Preferably, while the stationary rotor flux is being established,capacitors C1 and C2 are being charged, again with a unidirectionalcurrent. The derivation of the supply of capacitors C1 and C2 is notshown for convenience and case of illustration, but again is describedin more detail in McDonald. A preferred method, as described inMcDonald, is to charge the capacitors using the leakage inductance fromthe winding used to establish the stationary rotor flux so that adesired voltage can be achieved across the capacitors. However, thismethod of charging using the leakage inductance is not alwayspracticable, particularly for large scale motors for the followingreasons. Large scale motors tend to have small leakage inductances incontrast to their smaller counterparts. As a result, to chargecapacitors to a predefined voltage in large scale motors, would requirelarge transistors operating for a long time. Due to this longer chargingtime, transistors employed in the charging circuitry would theninvariably require some form of cooling (heat sinking) which in turnresults in adding extra components and hence adding cost to thecircuitry. Therefore, to overcome these problems associated with thecharging process described by McDonald, a transformer with a highvoltage winding is a preferred method in this invention.

In this invention Capacitor C1 is much larger than capacitor C2.Capacitor C2 is the one, which is chosen to be connected across thewinding between terms 8 and 10 during normal operation of the motor inthe known Steinmetz configuration.

Once capacitor C1 has been charged sufficiently (for example up tobetween 1,000 and 2,000 volts), and once the stationary rotor flux hasbeen established, switch S1 is opened and switch S2 (which may comprisean SCR device) is closed thereby applying the charge on capacitor C1across the winding between terminals 8 and 10. A current begins flowingthrough the winding between terminals 8 and 10 thereby energising thewinding, and generating a stator flux. This has the effect of causingthe rotor to try to align itself in relation to the field produced bythe stator winding. As the charge on the capacitor C1 is quite high, asignificant stator flux is generated which causes rapid acceleration ofthe rotor.

After a predetermined time period corresponding to a time when thecurrent supplied by capacitor C1 to the winding between terminals 8 and10 would be reducing, ie. most of the energy from the capacitor has beentransferred to the winding between terminals 8 and 10, switch S3 (againcomprising a device such as an SCR) is closed. Because capacitor C2 isstill charged, the effect of closing S3 results in S2 being reversebiased and therefore turning off. Capacitor C2 is now effectivelyconnected across the winding between terminals 8 and 10, and after avery short time period calculated to ensure that S1 has been turned off,switch S4 is turned on to connect the mans supply 4 between terminals 6and 10 (which connects the mains supply to the motor) so the motorcontinues to accelerate up to operating speed.

Turning now to FIG. 2, a diagrammatic plot of voltage and currentagainst time is shown. Firstly, considering the current IS2, being thecurrent through switch S2, it can be seen that current from thecapacitor begins to flow when switch S2 is closed, and as expected thecurrent increases rapidly as it energises the winding between terminals8 and 10 until a maximum is reached, after which the current begins todecline as energy is transferred to the winding both from the capacitorand from the rotating rotor, before increasing again.

As described above, normally, switch S3 will be closed shortly before,or near the minimum current flow IS2, and this is usually simplycalculated by allowing a predetermined time period to elapse fromclosing S2.

Now considering the voltage VM across the winding between terminals 8and 10, it will be seen that the voltage increases as the current flowincreases over time through the winding. The voltage VC1 across C1 willdecline overtime. However, if switch S3 is not closed, VC1 will notreturn to zero, but will instead stay at a level corresponding to theback EMF induced in the winding between terminals 8 and 10 as the rotorrotates relative to that winding. Accordingly, when switch S3 is closed,the voltage which will be preset on capacitor C1, will be the back EMFvoltage induced by movement of the rotor.

The induced back EMF voltage in a winding will be directly proportionalto the speed of rotation of the rotor relative to the stator. Therefore,the magnitude of the voltage on capacitor C1 when switch S2 is turnedoff, is the back EMF voltage, which will be directly proportional to thespeed of rotation of the rotor.

Accordingly, a measurement of the voltage on capacitor C1 when switch S2turns off, provides a reliable indication as to whether the rotor is infact rotating, and of the speed of rotation of the rotor, so that anindication as to whether the motor is in an appropriate condition forapplication of mains supply can be established.

Alternatively, the voltage across the winding between terminals 6 and 10can also be regarded as the back EMF voltage induced in the motorwinding, neglecting the voltage drop due to winding inductance andresistance. This voltage plotted against time is shown in FIG. 3, whereinstants t1 and t2 correspond to the moments switches S2 and S3 areclosed, respectively. This induced voltage, which delays down to zero ifmains supply 4 is not connected to the motor after closing switch 3, isdirectly proportional to the speed of the rotor and therefore as therotor slows down the period of this waveform increases as can be seenfrom FIG. 3. It is evident from the waveform that the ideal and earliestmoment for closing switch 4 in order to connect the mains supply to themotor would be the instant t3 as the voltage across the winding is thenessentially zero. Since the induced back EMF voltage in the winding is adirect indication of the speed of the rotor, an accurate estimate of therotor speed can be obtained by measuring the time between the initiationof rotation and the voltage reaching a predefined value (ie: zero backEMF voltage). If this measured time exceeds a predefined value (ie: ifthe initial staring speed of the rotor is below the predefined startingspeed), switch 4 will not be closed to connect the mains supply 4 to themotor.

Turning to FIG. 4, a comparator CP1 is shown. One input to thecomparator is derived from a measurement of the voltage across capacitorC1. The measurement of voltage on capacitor C1 is easily establishedbecause in practical implementation of starting circuits correspondingto that shown in FIG. 1, the voltage across capacitor C1 is measured inany event to measure the charge that is delivered to C1 before switch S2is closed. Therefore, the appropriate circuitry for providing ameasurement of the voltage on capacitor C1 will already be in place andit is a simple matter to supply this to comparator C1. Similarly, forany given motor, the back EMF generated in a winding by any particularspeed of rotor rotation is easily established and any selected signalwhich is indicative of an appropriate back EMF can be provided to theother input 20 to comparator C1. The output of comparator C1 may beprovided to a reset line in the control circuitry for example so that ifthe signal derived from the voltage across capacitor C1 does not exceedthe predetermined threshold value provided to input 20, then thestarting sequence is reset for another attempt. Alternatively, theoutput of capacitor C1 may be provided to a disable line, which mayestablish some form of alarm. Naturally, further logic circuitry couldbe provided so that after a certain number of attempts, an alarm orother indication will be provided to a user.

The selection of the predetermined back EMF may for example be 50% ofthe rated rotor speed. Therefore, if normally at full rotational speed aback EMF of 320 volts would be present across the winding, then thesignal at input 20 to the comparator may be representative of a back EMFof 160 volts.

Alternatively, referring to FIG. 3 again, the inputs to the comparatorcan also be the voltages at terminals 6 and 10. The output of thecompactor will then change as the polarity of the voltage across thewinding changes (ie: when the voltage across the winding goes through azero crossing). Therefore the speed of the rotor immediately beforeconnecting the mains supply to the motor, can thus be estimated bymeasuring the time to this moment from a known reference point in time.

Preferably, to ensure that the rotor has reached or exceeded thepredefined threshold (safe) speed before connecting the mains supply tothe motor, the starting procedure is carried-out twice every time themotor is connected to a new load. In the further attempt, the timebetween the first back EMF zero crossing after closing switch S3 and areference point is measured without connecting the mains supply to themotor. If the measured time period is below the predefined value, thestarting procedure is carried-out again assuming that the rotor willexceed the threshold speed in the second attempt and the mains supply isconnected to the motor by closing switch 4 after the measured timeperiod in the first attempt. However, if the measured capacitor voltageis below the predetermined value just prior to the moment of closingswitch S4 in the second attempt, the switch S4 will not be closed asthat indicates the rotor speed has not exceeded the predefined safestarting speed and hence the starting is discontinued.

This circuit described above has a number of advantages. Firstly, it isvery fast, so the circuitry ‘knows’ within one mains cycle whether ornot the motor has started. The alternative technique, as mentionedabove, is to measure the motor current when switch S4 is turned on andif that motor current is too high, then turning off switch 4 before anydamage is done. However, this has a disadvantage that the user has towait several cycles ie. hundreds of milliseconds, before the decisioncan be made. The current method and apparatus provide a technique whichis approximately two orders of magnitude faster than any othermeasurement that could be accurately made. It also has the advantagethat it is very low cost. As described above, the circuitry forproviding an indication of the voltage across capacitor C1 or voltageacross winding terminals 6 and 10, is already present, so in essence allthat is required is a comparator. However, it will be apparent to thoseskill in the art that the word techniques described above may beperformed using indirect methods, for example by not sensing the backEMF directly.

1. A starting circuit for an electric induction motor having a pluralityof phase windings to be energised from an electricity supply, the phasewindings having at least three terminals for connection to the supply,the circuit comprising a first switching means arranged and controllableto conduct a uni-directional current derived from the supply between afirst combination of the terminals to establish a stationary rotor fluxin the rotor of the motor, a second switching means arranged andcontrollable to supply a starting current between a second combinationof the terminals selected to generate a stator flux at an angle to thestationary rotor flux, switch control means to control the first switchmeans to establish the stationary rotor flux and to control the secondswitching means to initiate supply of the starting current to provide astarting torque for the motor, and back EMF sensing means to sense aback EMF generated by rotation of the rotor to sense whether the rotorhas turned sufficiently to be in a condition to connect the motor to thesupply.
 2. A starting circuit as claimed in claim 1 including a thirdswitching means arranged and controllable to connect the supply toprovide an operating supply to the terminals of the motor during normalrunning of the motor, and the switch control means controlling the thirdswitching means to provide the operating supply subsequent to thestarting torque and provided the back EMF sensing means sense that therotor has turned sufficiently to be in a condition to connect the motorto the supply.
 3. A starting circuit as claimed in claim 1 including anelectrical energy storage device and means to charge the storage device,the second switching means being arranged to supply the starting currentby discharging electrical energy from the storage device.
 4. A startingcircuit as claimed in claim 3 wherein the back EMF sensing meanscomprise voltage measuring means to measure the voltage across theenergy storage device when the second switching means is deactivated. 5.A starting circuit as claimed in claim 1 including comparison means tocompare the magnitude of the detected back EMF with a pre-determinedvalue to thereby ascertain whether the motor is in a condition forconnection of the operating supply.
 6. A starting circuit as claimed inclaim 1 wherein the back EMF detection means provide an indication ofthe magnitude of the back EMF over time.
 7. A starting circuit asclaimed in claim 6 wherein the comparison means compare a characteristicof the magnitude of the back EMF over a time period and compare thesensed time period with a predetermined time period to ascertain whetherthe motor is in a condition for connection to the operating supply.
 8. Astarting circuit as claimed in claim 5 wherein the comparison meanscomprise a comparator.
 9. A starting circuit as claimed in claim 2including an electrical energy storage device and means to charge thestorage device, the second switching means being arranged to supply thestarting current by discharging electrical energy from the storagedevice.
 10. A starting circuit as claimed in claim 9 wherein the backEMF sensing means comprise voltage measuring means to measure thevoltage across the energy storage device when the second switching meansis deactivated.
 11. A method of starting an electric induction motorhaving a plurality, of phase windings, the method comprising the stepsof delivering a uni-directional current to the motor to establish astationary rotor flux in the rotor of the motor, delivering a startingcurrent to the motor to produce a stator flux at an angle to thestationary rotor flux to produce a motor starting torque, detecting theback EMF produced by rotation of the rotor in response to the startingtorque, and determining whether one or more characteristics of thedetected back EMF indicates that the motor is in a condition forconnection to an operating supply.
 12. A method as claimed in claim 11including the step of connecting the motor to the operating supply ifone or more characteristics of the detected back EMF indicates that themotor is in a condition for connection to the operating supply.
 13. Amethod as claimed in claim 11 wherein the back EMF is measured from amotor winding.
 14. A method as claimed in claim 11 wherein the energyfor the starting current is accumulated in an energy storage deviceprior to delivery of the starting current.
 15. A method as claimed inclaim 14 wherein the back EMF is detected by measuring the voltageacross the energy storage device following disconnection of the energystorage device.
 16. A method as claimed in claim 14 wherein acharacteristic of the back EMF of a motor winding is sensed over time tosense the time period over which the characteristic is repeated, and thesensed time period is compared with a predetermined time period todetermine whether the motor is in a condition for starting.
 17. A methodas claimed in claim 16 wherein, if the motor is in a condition forstarting, the method steps of delivering a uni-directional current to awinding, delivering a starting current and sensing the back EMF arerepeated, and the back EMF is measured by detecting the voltage acrossthe energy storage device after disconnection of the energy storagedevice, and if the voltage across the energy storage device exceeds apredetermined value, connecting the motor to the operating supply.